May. 05, 2025
Hydraulic systems play a critical role in various industries, powering machinery and equipment with precision and efficiency. These systems are integral to operations in industries such as Oil & Gas, Cranes & Aerial Lifts, Construction, Forestry, Agriculture and Mining, where they enable the movement of heavy loads, the operation of machinery, and the execution of complex mechanical tasks. At the heart of these systems lie hydraulic cylinders, converting fluid power into mechanical force allowing for controlled and powerful movements which are essential to the functionality of equipment in diverse industrial applications. An integral element to these systems is the Hydraulic Cylinder Seals.
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A key component to ensuring the optimal performance of these cylinders is a hydraulic seal. This small but mighty piece of the cylinder is essential for maintaining the integrity and efficiency of the entire system. They prevent leaks by containing the hydraulic fluid within the system, ensuring the pressure needed for operation is maintained. They also keep contaminants out of the cylinder, which is important for protecting its internal components from damage and wear. The reliability and longevity of hydraulic systems are heavily dependent on the quality and performance of seals.
At RHK Hydraulic Cylinder Services Inc., we pride ourselves on being industry leaders in complete hydraulic cylinder manufacturing, service and repair. With more than three decades of experience, we provide top-notch solutions tailored to meet each project’s specific needs. Our expertise in custom cylinder design and our deep understanding of hydraulic systems enable us to deliver products and services that ensure top performance and longevity of your equipment. We’re dedicated to ensuring that every hydraulic cylinder we service and manufacture meets the highest standards of quality and reliability.
The operating environment significantly influences the choice of seal material. For instance, in applications exposed to abrasive particles or harsh chemicals, seals made from durable and chemically resistant materials such as PTFE or polyurethane are preferable. Polyurethane seals are ideal for environments with high abrasion, such as mining and construction, due to their excellent wear resistance. PTFE seals offer superior chemical resistance, making them suitable for applications involving aggressive chemicals or solvents. In contrast, nitrile rubber seals are suitable for environments with moderate temperatures and exposure to hydraulic fluids.
Hydraulic cylinders operate under varying pressure and temperature conditions, affecting seal performance. High-pressure applications require seals with excellent strength and abrasion resistance, such as polyurethane. For high-temperature environments, PTFE seals are ideal due to their stability and low friction properties. Polyurethane seals can withstand high pressures and provide excellent wear resistance, making them suitable for heavy-duty applications. PTFE seals maintain their properties at high temperatures, making them ideal for applications in extreme heat, such as in foundries or steel mills.
The hydraulic fluid used in the system also determines the suitable seal material. Nitrile rubber seals are compatible with most hydraulic fluids, making them a versatile choice. However, for systems using specialized fluids or operating in extreme conditions, PTFE or polyurethane seals offer enhanced performance and longevity. Nitrile rubber seals are compatible with a wide range of standard hydraulic fluids, making them suitable for general-purpose applications. On the other hand, PTFE seals offer excellent chemical resistance and are suitable for systems using specialized synthetic or bio-based hydraulic fluids.
Hydraulic cylinders rely on several types of seals, each serving a specific purpose. Among the most common are piston seals and rod seals.
Piston seals, also known as piston rings, are located in the cylinder barrel, creating a seal between the piston and the cylinder bore. Their primary function is to contain the pressurized hydraulic fluid on one side of the piston, allowing the cylinder to generate force in a specific direction. Piston seals must withstand high pressure and dynamic movement, making the material they’re made critical to the overall functionality of the hydraulic cylinder.
There are two common types of piston seals used in hydraulic cylinders:
Single-Acting Seals: When it comes to single-acting seals, they are designed for cylinders that apply force in one direction. They’re typically used in applications where the piston only moves in one direction and returns to its original position via external force or spring.
Double-acting seals: These seals are used in cylinders that apply force in both directions. They must withstand pressure from both sides of the piston, requiring materials with strong wear resistance and durability.
Rod seals, located in the gland or seal housing, prevent hydraulic fluid from leaking out of the cylinder and protect the internal components from contamination. These seals must perform under dynamic conditions, withstanding the reciprocating motion of the rod. The effectiveness of rod seals directly impacts the cylinder’s efficiency and the overall hydraulic system’s performance. There are also two types of rod seals:
Lip seals: They feature a flexible lip that creates a seal against the rod, preventing fluid leakage. They are commonly used in low to medium-pressure applications.
U-Cup Seals: They have a U-shaped cross-section that provides a tight seal against the rod. They are best suited for high-pressure applications and provide excellent wear resistance.
Selecting the appropriate seal material is crucial for the optimal performance of hydraulic cylinders. The choice depends on various factors, including the operating environment, pressure, temperature, and fluid compatibility.
Contamination is a common issue in hydraulic systems, leading to premature wear, reduced efficiency, and potential failure. Wiper seals, also known as scrapers, play a vital role in preventing contaminants from entering the cylinder and damaging internal components.
Wiper seals are installed on the external side of the cylinder, where the rod exits the housing. Their primary function is to scrape off dirt, dust, and other debris from the rod as it retracts into the cylinder. By preventing contaminants from entering the hydraulic system, wiper seals enhance the longevity and reliability of the cylinder.
Wiper seals are typically made from durable materials like polyurethane and nitrile rubber. Polyurethane wipers offer excellent abrasion resistance, making them suitable for applications with heavy contamination. These are ideal for harsh environments with high levels of contamination, such as construction and agriculture, due to their excellent wear resistance. Nitrile rubber wipers provide good flexibility and are effective in environments with moderate contamination levels. These are suitable for applications with moderate contamination, such as industrial machinery, due to their flexibility and resistance to hydraulic fluids.
The performance and longevity of hydraulic seals directly impacts the efficiency and reliability of hydraulic systems. Proper maintenance and care are essential to maximize seal life and ensure optimal performance.
Regular inspections of hydraulic seals helps to identify signs of wear, damage, or contamination early. Visual checks for cracks, deformation, or fluid leaks can prevent minor issues from escalating into major failures. RHK recommends that users regularly check seals for signs of wear, cracks, or deformation to identify potential issues early. Scheduled maintenance, including cleaning and lubrication, ensures seals remain in good condition and perform effectively. We also recommend that you implement a routine maintenance schedule that includes cleaning and lubrication to keep seals in optimal condition.
Proper installation of hydraulic seals is crucial for their performance. Incorrect installation can lead to premature wear, leaks, and system inefficiencies. Following manufacturer guidelines and use of appropriate tools during installation ensures seals are correctly positioned and function as intended.
Hydraulic cylinder seals are critical components to ensure the efficiency, reliability, and longevity of hydraulic powered systems. Understanding the different types of seals, their unique roles, and the importance of material selection is key to optimizing equipment performance. By preventing contamination and ensuring proper maintenance, hydraulic seals can significantly enhance the lifespan of hydraulic cylinders.
For those seeking expert guidance and top-quality hydraulic seals, RHK Hydraulics offers comprehensive hydraulic cylinder services, custom hydraulic cylinder manufacturing, and repair solutions. As one of the leading hydraulic cylinder manufacturers in Canada, RHK Hydraulic Cylinder Services Inc. is dedicated to providing reliable and durable hydraulic solutions tailored to meet the unique needs for each application.
Hydraulic cylinders are vital to various industrial applications, and ensuring they operate efficiently and reliably is essential. By understanding the different types of hydraulic cylinder seals and how to maintain them, you can optimize the performance of your businesses’ hydraulic systems. Investing in high-quality seals and following best practices for maintenance and installation will help you achieve long-lasting, reliable operations of your equipment.
This chapter delves into hydraulic seals, examining their design and construction, as well as the essential factors in selecting the right seal for the purpose.
Hydraulic seals are specialized ring-shaped gaskets intended to bridge gaps between hydraulic cylinder components. They are vital for stopping fluid leaks at these junctions. Each seal is uniquely designed to fit the particular sections of a hydraulic cylinder, ensuring a dependable, leak-free connection.
Although O-rings and hydraulic seals have similarities, hydraulic seals are distinguished by a groove on the lip that ensures secure component attachment. This design feature guarantees a leak-tight seal even as it glides over components. The seal maintains containment of fluids within the designated chamber, even when piston movement and fluid pressure in the cylinder rise.
In the construction of hydraulic seals, both the manufacturing process and material selection are critical considerations.
The production of hydraulic seals involves Computerized Numerical Control (CNC) lathe machines. These advanced machines create both standard and bespoke seal profiles by being programmed with digital profiles, then cutting the seal from the selected material.
Hydraulic seals can be crafted from a variety of materials. Rubber is frequently used for its flexibility, resilience, and crack resistance. Polyurethane is also favored, being more durable and wear-resistant than rubber while offering equivalent functionality.
Another less typical material for hydraulic seals is Polytetrafluoroethylene (PTFE), known for its pliability, durability, and high-temperature resistance. The choice of material hinges on specific operating factors such as fluid type, pressure, chemical compatibility, and temperature conditions.
Consider the following factors when selecting a seal:
The shaft's peak speed is influenced by elements like shaft finish, runout, bore alignment, fluid type, and the oil seal material.
The operational temperature range of the mechanism must remain within the limits of the seal's elastomer temperature specifications.
Typical oil seals are suited for low pressures, generally around 8 psi or less. For systems experiencing higher internal pressure, a pressure relief mechanism is essential.
A shaft with a Rockwell hardness of 30 or higher can expect longer seal life. For abrasive environments, increase the hardness to RC 60.
Superior sealing efficacy is achieved with excellent shaft surface treatments. The seal's efficacy is impacted by spiral lead and the direction of tool mark finishes. For optimal sealing, shafts should be polished or ground with concentric finish marks, avoiding spiral lead. If necessary, align spiral finishes with the fluid flow as the shaft rotates.
If bore and shaft centers are misaligned, seal lifespan reduces due to uneven wear on the sealing lip.
Maximum seal effectiveness is achieved when shaft and bore tolerances are closely aligned. Account for factors like shaft eccentricity, end play, and vibration.
Keep runout to a minimum. Wobbling bearings or shaft whip commonly cause rotation center movement. The effect is worsened with misalignment, contrary to popular belief, flexible couplings do not always compensate for misalignment.
Regular lubrication with oil of suitable viscosity improves seal performance and longevity. Check for seal compatibility with additives and synthetic oils to prevent potential issues.
The various types of hydraulic seals are:
Hydraulic cylinder seals seal openings between the various parts of the hydraulic cylinder. Their design makes them retain hydraulic fluids, keep solid or liquid impurities out, and maintain hydraulic pressure. These tasks require a wide range of seal designs and performance-boosting features.
To ensure fluid containment, the seal material must conform to any irregularities on the metal surfaces. The seal needs to adjust quickly by expanding or compressing to accommodate changes in clearance gap size. Additionally, the seal should possess adequate modulus and hardness to withstand shear stress from system pressure, preventing extrusion into gaps.
Effective sealing in fluid power systems requires not only containing the fluid but also keeping contaminants out. The choice of seal depends on the nature of the surfaces it interacts with. Static seals are used when there is no movement between the surfaces, while dynamic seals are necessary when there is relative motion, such as oscillating or reciprocating actions.
The design of the seal lip varies according to its intended application. Most seals feature what are referred to as "lips" in their construction, including those used in radial, rotary, and linear shaft seals.
Lip seals not only serve as barriers or dams but also act as pumps. They are frequently used with rotary, reciprocating, and oscillating shafts. The primary roles of a lip seal, also known as a radial or rotary seal, include retaining lubricants, keeping contaminants out, maintaining pressure, and separating fluids.
Wiper scraper seals inherently include lips in their design, classifying them as lip seals. However, the term "lip seal" lacks a standardized definition. Additionally, radial seals, often called lip seals, are referred to as rotary seals in the UK. Although "rotary" is technically accurate, "radial" is more commonly used in the US because these seals are radially energized (typically with a spring) and are smaller than the shaft they encase.
The more popular European name for this type of seal – rotary shaft seal – is selected since it is most usually utilized where a rotating shaft passes through it. Rotary seals, which are similar but slightly different, are also utilized in linear applications like motorcycle fork stanchion sliders. These necessitate a whole distinct lip design, leading to a misunderstanding of the entire rotational terminology. These misunderstandings can lead to application issues.
The primary role of a lip seal is to prevent contaminants from entering while retaining lubricants. By design, lip seals generate friction, making them suitable for various applications, from slow-moving machinery to high-speed rotations, and across a broad temperature range from below zero to over 500 degrees Fahrenheit.
For a lip seal to be effective, it must maintain proper contact with its rotating counterpart, which depends on correct selection, installation, and maintenance. New lip seals might leak initially due to improper installation, but some may cease leaking once the material conforms to the shaft.
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Historically, a basic lip seal was a leather strap fitted around a wheel axle. Modern lip seals are influenced by various factors and come in different types, including non-spring and spring-loaded versions, as well as various contact patterns. Non-spring seals are generally more affordable and effective at retaining viscous substances like oil at lower shaft speeds. They are commonly used in applications such as conveyors, vehicle wheels, and lubricated components. On the other hand, spring-loaded seals are widely used across various equipment and are typically employed with oils.
Mechanical seals are used in rotating equipment like pumps and mixers to prevent leaks of liquids and gases into the environment.
A mechanical seal consists of two primary components: one stationary and one rotating against it to form the seal. These seals vary widely in shape and complexity, from basic single-spring designs to intricate cartridge models. The design, materials, and construction depend on factors like pressure, temperature, rotational speed, and the type of product being sealed.
A typical mechanical seal consists of seven key components: the stationary component (often referred to as the seat), the stationary component sealing member, the rotating component, the rotating component sealing member, the spring, the gland plate, and the clamp ring.
Mechanical seals feature four primary sealing points. The primary seal is located between the rotating and stationary faces. The gasket seals between the stationary member and the stuffing box face. The secondary seal, which may be an o-ring, is positioned between the rotating member and the shaft or shaft sleeve. Lastly, the seal between the gland plate and the stuffing box is typically achieved with a gasket or o-ring.
The key sealing point between the rotating and stationary components is crucial to the mechanical seal's design and function. Typically, spring force presses the rotating and stationary faces together.
These mating faces are meticulously machined (lapped) to achieve extreme flatness, often within 2 light-bands (an optical measurement standard). This precision minimizes leakage to almost negligible levels. However, some minimal leakage may still occur and manifest as a mist. Spring compression usually provides the initial face pressure, and when the seal is idle, the springs maintain this pressure to prevent leakage.
Without lubrication, the friction between the seal faces and the resulting heat would lead to wear and eventual seal failure. Therefore, lubrication is essential, typically provided by the product medium. This lubrication forms a fluid film, which is crucial for maintaining the seal’s performance and ensuring reliable and consistent operation.
Oil seals, also referred to as grease, fluid, and dirt seals, are designed to cover gaps between stationary and moving parts in mechanical equipment. They serve to prevent lubricants from leaking out and to keep contaminants from entering the machinery, which is particularly important in challenging environments. Additionally, oil seals help to prevent the mixing of different fluids, such as lubricating oil and water.
Custom seals can be tailored for new machinery to fit specific bearings, providing protection for various types of precision bearings such as ball, sleeve, and roller bearings in nearly all types of equipment, including vehicles. The seal's structure is reinforced by an internal metal ring that acts as a supportive core.
The outer layer of the seal is made from a flexible material like nitrile rubber or other substances based on the operating conditions. A spring supports the seal's lip to prevent lubricant leakage, while the lip design helps keep contaminants out.
For light load applications, the outer skin may be made of silicone. To withstand high temperatures (above 120 degrees Celsius), fluoroelastomer (Viton) is used. Alternatives such as Poly Acrylate or Polytetrafluoroethylene (PTFE) can also be employed.
The shaft where the oil seal is mounted must have a rough surface finish. It should be toughened to avoid grooves that could form under the pressure of the seal's spring. Additionally, the installation area must be ground to prevent grooves that could accelerate wear on the seal’s lip.
To prevent leakage, oil seals have a flexible lip that brushes against the rotating shaft or housing. The lip is kept in contact with the shaft by the spring. Dynamic seals with a rotor or rotating member and a stator or stationary member are known as bearing isolator oil seals. The rotor rotates in tandem with the shaft. Bearing isolators with a "labyrinth" construction are used in some oil shafts. Others use O-rings that are less complicated.
Oil seals are rotary shaft seals designed to close the gap between moving and stationary components, preventing lubricant escape and stopping contamination through the clearance. They come in various types and materials to suit different applications and environments.
The appropriate material and type of oil seal depend on the medium used and the application. Typically, a standard oil seal features a metal ring as its inner skeleton for structural stability, while the outer part may be made of metal or rubber based on the specific requirements.
The spring on the oil seal’s lip supports the lip, preventing lubricant leakage and blocking contaminants. When a dust lip is present, it protects the sealing lip from dirt and dust, extending the seal’s lifespan.
Dust lips are located on the inner diameter of the seal, and a seal with a dust lip is known as a double lip oil seal. Additionally, a garter spring, a coiled spring shaped into a circle, is often used to maintain radial force exerted by the sealing lip around the shaft surface.
Radial oil seals, the most common type, function by forming a thin film of oil between the shaft and the rubber sealing lip. This oil layer lifts the sealing lip slightly away from the shaft, creating a barrier that prevents oil leakage past the lip. As a result, rubber oil seals are not ideal for dry running applications or high-pressure environments.
The most frequently used types of oil seals are those with metal and rubber casings.
Metal-cased oil seals are used when installed in a housing bore made from the same material, allowing for equal expansion and contraction during operation and thereby preventing leakage. Typically, metal-cased seals are more cost-effective compared to rubber seals.
Rubber-cased oil seals are often used when metal-cased seals might fail, such as due to thermal expansion. Unlike metal-cased seals, rubber-cased seals do not rust and can better accommodate slightly damaged housings. Rubber expands quickly under high pressures and temperatures, providing a tight and stable seal.
The Type R seal is one of the most commonly used types. It features a carbon steel insert with a rubber outer diameter, offering excellent sealing even with a slightly out-of-tolerance housing. The sealing lip, reinforced by a spring, ensures effective shaft sealing and allows for press-fitting in the housing with sufficient interference for static sealing.
Made from high-performance nitrile rubber and featuring a high-quality galvanized steel garter, this seal is designed for optimal durability. To prevent leakage caused by hydrodynamic pumping, it is crucial that the contact area of the sealing lip on the shaft or sleeve is free from any machine lay traces.
Oil seals can feature various lip designs, including:
This design incorporates a garter spring and is typically used for sealing against internal media in lower pressure applications. Single lip seals are not recommended for environments with contaminants or dirt.
Similar to the single lip design, the double lip seal also uses a garter spring but includes a primary sealing lip that addresses internal media in lower pressure situations. It provides enhanced sealing capability compared to the single lip design.
Rod seals are essential components in fluid power equipment, crucial for preventing fluid leakage from inside the cylinder to the external environment.
Rod seals are primarily made from PTFE blends and polyurethane.
Leakage through a rod seal can decrease equipment performance and potentially lead to environmental issues. Optimal sealing performance requires perfect pairing of the rod seal with the wiper. If an aggressive rod seal is matched with an aggressive wiper, the wiper may scrape off the thin film of oil left on the rod during the return stroke, leading to system leakage.
Various hydraulic rod seals are available for both single-acting and double-acting systems. Rod seals also play a role in preventing environmental contamination when combined with a wiper seal.
Rotary seals are designed for applications with a rotating shaft in wet environments. They keep lubricants (such as grease, oil, or water) contained while preventing dirt and water ingress. These seals are crucial for protecting critical components in pumps, ships, and tidal turbines from fluid damage.
Rotary seals encompass a variety of types, including rotary shaft seals, radial oil seals, double-acting O-ring energized polytetrafluoroethylene seals for bore and shaft, radial and axial lip seals, rubber V-rings, mechanical face seals, and others.
These seals are known for their low-friction properties and excellent wear resistance. They effectively prevent corrosive moisture, abrasives, and other contaminants from entering machinery and also keep different mediums, such as water and lubricating oil, from mixing. Rotary seals are typically made by vulcanizing an elastomer onto a metal ring.
Rubber seals are ideal for reducing vibration and noise. They include various designs such as co-extruded pedestals with sponge rubber bulbs, ribbed profiles, lid seals, and triangular sections.
Rubber seals are straightforward to install and find applications in the automotive, marine, industrial, and manufacturing sectors.
Rubber seals can be made from various materials, including:
This synthetic rubber, known as chloroprene or polychloroprene, is created through polymerization. It is highly versatile and suitable for use in diverse, harsh conditions across many industries due to its resistance to alkalis, acids, oils, greases, sunlight, ozone, and weathering. Its excellent flexibility and resistance to twisting make it ideal for hydraulic rubber seals, which inherit these robust properties.
This elastomeric material is widely used for seals due to its excellent resistance to alkalis, acids, gasoline, petroleum-based compounds, and hydraulic fluids. It is particularly well-suited for hydraulic seals where water permeability and abrasion resistance are critical concerns.
This high-performance elastomer is known for its exceptional temperature stability, withstanding extremes from -75 degrees Fahrenheit to +500 degrees Fahrenheit. It also resists damage from oxygen, ozone, aging, water, and weathering.
This high-performance rubber material boasts excellent chemical resistance and can endure high temperatures. It has strong tensile strength and a low compression set, making it suitable for use in temperatures ranging from -15 degrees Fahrenheit to +400 degrees Fahrenheit, with intermittent exposure up to +500 degrees Fahrenheit.
This section will cover the applications of hydraulic seals, their advantages, and typical failure modes.
The common failures of hydraulic seals are:
Hydraulic seals can harden when subjected to high temperatures, which may result from either rapid heat generation during stroking operations or elevated fluid operating temperatures. As the seals lose their elasticity and develop cracks, they ultimately fail.
Significant damage to a seal can occur from wear on the dynamic face of the seal lip, often due to excessive lateral load or inadequate lubrication.
The lifespan of seals is significantly influenced by the tools and methods used during installation. Improper installation can cause cuts or dents on the dynamic lip of the seal, affecting its efficiency and potentially introducing contaminants into the hydraulic fluid.
Seal fractures involve breakage, bending, long cracks, or complete separation of the seal's dynamic side. This damage is often due to excessive backpressure, high-pressure shocks, or the use of substandard materials in seal manufacturing.
Improper installation can lead to various issues with hydraulic seals, including contamination, unsafe handling, and incorrect seal sizing. It is crucial to ensure that the design is correctly executed before constructing the seal to achieve proper sealing performance.
Contamination occurs when external debris, such as mud, dirt, or fine particles, enters the hydraulic rod. These particles can adhere to the piston and cause the seal to become dirty, compromising its ability to effectively keep contaminants away from the piston area.
Chemical erosion happens when seal material is exposed to corrosive fluids. This problem arises from using inappropriate seal materials for the specific application. Chemical attacks from hydrolysis, oil additives, or oxidation can lead to the degradation of the seal, including loss of the seal lip interface, swelling, softening, or shrinkage. Discoloration of the seal often indicates chemical erosion.
Hydraulic seals serve to prevent the leakage of fluid from within a system to the outside. There are different types of hydraulic seals, each offering its own unique properties. The properties of the hydraulic seals are dependent on the type of material as well as the design. Therefore when opting for a hydraulic seal, careful considerations must be made to ensure that a seal that perfectly suits a particular application is selected. The considerations that can be investigated prior to selecting any seals are fluid pressure range, temperature range, stroke speed, fluid type, hardware dimensions, and cylinder application.
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